Analysis of spin-lattice relaxation ( T I ) provides alternatives to the standard line shape procedures for the investigation of intramolecular reactions; furthermore it expands the NMR range at both the high and the low energy barrier limits. Dipole-dipole-and quadrupole-relaxation times are sensitive to very rapid processes such as methyl rotation. Analysis of relaxation in the rotating frame (T,& provides kinetics for many sorts of processes, particularly those in the dynamic range that is too rapid for line shape methods. Saturation transfer and the coalescence of relaxation times at high temperatures can be used to measure high energy barriers. The scope and limitations of these methods are described.

determined previously by Long et al.11 The order tensor elements have been computed by setting r23 to a value of 2.4785 the microwave value for fluorobenzene and 2.481 for the remaining halobenzenes.Our recent determination of the signs of the order parameters of the laurate amphiphile by carbon-13 shielding anisotropy measurements indicates that the long axis of the surfactant is perpendicular to the applied field direction (see ref 25). These results are not consistent with a cylindrical superstructure for the potassium laurate type II nematic phase oriented perpendicular to the applied field. The order tensor elements for the halobenzenes show that the C2v symmetry axis is also on the average perpendicular to the applied field and coincident with the long (25) R. C.

Eine z.B. p‐Substitution durch Halogen bzw. eine Methyl‐, Methoxy‐ oder Methylthiogruppe führt zu einer Abhängigkeit von 1H‐ und 13C‐NMR‐Daten sowie der pKa‐Werte in.

Substitution in the aryl ring of 2-and 3-arylindenes has been used to examine the acidity of these hydrocarbons and the charge density and ion pair structure of their anions. Acidities were measured for the 2-arylindenes both in pure Me^O and in a Me2S0-H20 mixture. After allowance for the different acidities of the indicators used in the two media, the acidities of the indenes are higher in pure Me2SO (the pK.'s are lower by 1-2 units). The dispersion forces of Me2SO are more effective in stabilizing the delocalized charge of the anion than are the more localized interactions of the hydroxyl groups in the mixed solvent. The pK,'s in both solvents exhibit a linear response to the Hammett constant. Substituents that are electron withdrawing through resonance are better correlated by the " constant, suggestive of a strong resonance interaction between the aryl group and the indene ring. Such an interaction is substantiated by the chemical shift changes on deprotonation. The resonance of H4< (para position) in the unsubstituted 2-arylindenyl anion and the 13C resonance of C4. in all the 2-arylindenyl anions are shifted upheld in comparison to the neutral hydrocarbons, as a result of negative charge delocalization into the aryl ring. The sigmoidal shape of the plot of 13C chemical shifts of certain atoms in the 2-arylindenyl anions vs. gives evidence for a structural change within the anion. Only the carbons with very high charge density exhibit this behavior (Ci,3). Observations in the indene series generally parallel those in the l-aryl-4-phenylcyclopentadienes. The annelated phenyl ring in the indenes therefore causes effects similar to those of the simply bound phenyl ring in the cyclopentadienes.

The nature of the interaction between methyl and a trigonal carbon has been examined by the effect of substituents on the methyl rotational barrier. Barriers have been measured for para-substituted toluenes and for cisand trans-substituted propenes by the motional effects of methyl rotation on dipole-dipole spin-lattice relaxation. The toluene barriers exhibit a fair correlation with and a very poor one with . Thus hyperconjugation cannot be a major factor in determining the methyl rotational barrier. The propene barriers, particularly in the cis series, also correlate with but have a better correlation with ctr than do the toluenes. Examination of all the 13C chemical shifts showed that the rotational barriers correlate only with the ortho carbon in the toluenes and with the 2-carbon (methyl substituted) in the propenes. These results suggest that the methyl rotational barrier is primarily sensitive to the nature of the ortho C-H bond in the toluenes and of the -C-H bond in the propenes. The an and o¡ correlations are in accord with this model, since the ortho toluene carbon cannot interact directly through resonance with the para substituent but must depend on polar interactions. In the propenes, on the other hand, electron density at the -carbon is determined by both inductive and resonance effects. The major factor in determining these barriers is the electron density at the critical carbon center, which is the ortho carbon for the toluenes and the ct-carbon for the propenes.